(a) Field of the Invention
The present invention relates to an optical communication node system, an all-optical packet routing system, and an all-optical packet routing method and an all-optical packet routing network using the optical communication node system and the all-optical packet routing system. More specifically, the present invention relates to a system and method that transmits optical packets generated from one source communication node system to a destination communication node system without optical-to-electrical or electrical-to-optical conversion in a metro core network having a plurality of communication node systems connected thereto.
(b) Description of the Related Art
With the introduction of optical transmission technologies in a subscriber access network, there has been a tendency towards relieving restriction on the communication distance and hence expanding the service area of the access network to a metro access area. Recently, many studies have been made on the metro core network technology including RPR (Resilient Packet Ring) and light SDH (Synchronous Digital Hierarchy) for connections of different LANs (Local Area Networks) in the metro access area.
As the average communication bandwidth per user increases with the spread of FTTH (Fiber To The Home) communication, an increase in the traffic capacity at the subscriber network is expected to gradually accelerate. Hence, a high-capacity and high-speed packet switching function is needed for switching nodes of the metro core network that matches and connects the subscriber networks.
On the other hand, the all-optical packet routing technology without optical-electrical conversion is being watched with keen interest as an approach to ultimately intensify the metro core network, based on an analysis that the all-optical packet routing technology may enhance the economy of net building cost. The NTT (Nippon Telegraph and Telephone) suggested an all-optical packet routing optical subscriber network that transfers packets in the units of wavelength using AWG (Arrayed Waveguide Grating) [Ref: ISSLS2002]. If not opening the method to the public, the BTexact adapts the all-optical packet routing technology for the economy of the subscriber network.
All-optical packet routing has been so far considered as impractical because there is no optical buffer (optical memory), and OBS (Optical Burst Switching) technology has been studied to avoiding use of optical buffers.
However, this technology is still in the early state of research and is considered to be applicable only to extremely limited applications due to its unsolvable problems when actually applied to the network.
Until now, various ideas for implementation of the all-optical switching technology have been presented in literature at the experimental level. These approaches are mostly confined to a basic method of optical packet switching rather than implementation of systems or networks.
In recent years, a few studies have been made on an all-optical routing method of optical packets using a combination of existing optical technologies realizable in a network comprised of a plurality of communication nodes. The related art was disclosed in a paper recently presented at the ISSLS2002 by the NTT, which suggests a method of all-optical routing of optical packets from an OLT (Optical Line Terminal) located at a CO (Central Office) for the use purpose in a metro core network to an ONU (Optical Line Unit) adjacent to the user. This method, passing tests on a test bed, is considered as most realizable, as compared with the existing methods presented at various experimental levels.
FIG. 1 schematically illustrates the concept of the prior art.
The prior art of FIG. 1 is a typical WDM-PON (Wavelength Division Multiplex-Passive Optical Network) structure. An OLT sends n wavelength signals towards ONUs, and a WDM router (or WGR (Waveguide Grating Router)) routinely distributes assigned wavelengths to respective ONUs according to AWG routing mechanism. Uplink wavelengths transferred from ONUs to OLT differ from downlink wavelengths transferred from OLT to ONUs. The respective uplink wavelengths are designated to each of ONUs. Different uplink wavelengths are multiplexed at the WGR and sent to the OLT.
The downlink communication, which is achieved in the units of a super frame containing 32 frames, is a communication method of transferring a corresponding frame to a desired ONU by designating wavelengths by frames in the super frame and routinely distributing the designated wavelengths by the AWG. In the paper, the total number of wavelengths for 16 ONUs is 32, i.e., 16 downlink wavelengths (channels) plus 16 uplink wavelengths, and the number of frames assigned to a channel in the super frame is at most 8. If uniformly distributed, two frames per channel are allocated. The downlink has a data rate of 1.9907 Gbps, and for 32 frames, the data rate per frame is 62.2 Mbps (=1.9907 Gbps÷32). Hence, the data rate per channel is variable in the range of 62.2 to 497.7 Mbps in the units of 62.6 Mbps. Contrarily, the uplink has a fixed data rate of 497.7 Mbps per wavelength.
The ONUs use a light source having a fixed wavelength for uplink communication, but the OLT uses a light source having a high-speed wavelength converting. For this purpose, the paper proposes the use of an SSG-DBR (Super Structure Grating Distributed Bragg Reflector) LD (Laser Diode) that has a wavelength tuning range of 30 nm and a wavelength converting speed of 10 to 100 ns.
The prior art of this method is an all-optical structure that combines a high-speed wavelength conversion function of the OLT with a passive wavelength distribution function of the AWG to distribute packets to the respective ONUs in a simple way. This structure has a function of changing the downlink bandwidth to the ONUs dynamically within a defined range.
Compared with the general WDM-PON structure that connects the OLT to the individual ONUs with designated uplink/downlink wavelengths in the form of a dedicated path, the structure of the prior art divides the OLT output bandwidth by the number of ONUs connected to the OLT to determine an average downlink bandwidth per ONU. So, the structure uses the same number of wavelengths as the general WDM-PON structure but has a greater reduction of the downlink bandwidth.
The prior art is characterized in that uplink traffic is greater than downlink traffic, while the Internet service network usually has downlink traffic several to several scores of times greater than uplink traffic. Compared with the general WDM-PON in which the transmission rate per wavelength can be raised up to 10 Gbps at the current technological level, the prior art has the difficulty in packet transmission in a more than Gbps level because of the restriction on the wavelength change rate.
Moreover, the prior art is not practical in the aspect of cost, because an expensive wavelength-tunable light source just out on the market is used to make the downlink bandwidth variable for the packet transferred to the ONUs.